1. Field of the Invention
The present invention relates to a novel method of recording information by the use of a laser beam of highenergy density and an novel information recording medium advantageously employed for the recording method. More particularly, the invention relates to a method of recording information based on change of phase condition (i.e., phase change) and an information recording medium capable of recording and erasing information on the basis of the phase change.
2. Description of Prior Art
Information recording media utilizing a beam of high energy density such as laser beam have been developed in recent years and are put to practical use. Such recording medium is called an optical disc and its practical applications have been found, for example, as video disc and audio disc as well as disc memory for large-capacity computer and large-capacity static image file.
The optical disc basically comprises a disc-shaped transparent substrate of a plastic or glass material and a recording layer provided on the substrate. As a material of the recording layer, there are known a metal such as Bi, Sn, In or Te, a semi-metal, and a dye such as cyanine dye, metal complex dye or quinoline dye. Writing of information on the optical disc can be conducted, for example, by irradiating the optical disc with laser beam. Under irradiation with the laser beam, the irradiated area of the recording layer of the optical disc absorbs energy of the beam and a rise in temperature locally occurs, and as a result a chemical or physical change is caused to alter (or change) optical characteristics of the recording layer in the irradiated area, whereby the recording of information can be made.
The optical characteristics of the recording layer is altered, for example, by forming pits or protruded portions on the surface of the recording layer or forming bubbles in the recording layer, and the recording of information utilizing the formation of pits is widely employed. Otherwise, a method of utilizing a reaction between two recording layers, a method of utilizing a phase change and a method of utilizing magnetic inversion are also known for recording information. As the method of utilizing a phase change, there is known a process comprising elevating a temperature of a non-crystalline recording layer of As-Te-Ge type to higher than a glass transition point to cyrstallize the layer so as to perform recording of information.
Reading of information from the optical disc is also conducted by irradiating the optical disc with laser beam. The information can be reproduced by detecting reflected light or transmitted light corresponding to the change in the optical characteristics of the recording layer.
The optical disc includes two types, that is, one capable of recording (or writing) information only once (i.e., read only type) and the other capable of recording and erasing information repeatedly (i.e., erasable type).
Erasure of information from the optical disc can be done only in the case that recording of information is attained through reversible change of the recording layer. Accordingly, an optical disc which records information through change of shape (i.e., physical structure) of the recording layer such as formation of pits, etc. is of a read only type.
As a material of the recording layer, an inorganic material (e.g., a metal or a semi-metal) and a dye are heretofore used as described before, and any organic polymer material which is excellent in various properties such as workability, safety and cost had not been proposed up to recently. Recently, however, there is proposed an information recording method comprising using a polymer blend composed of a vinylidene fluoride polymer and polymethyl methacrylate as a material of the recording layer and utilizing change of phase condition of the polymer blend between a homogeneous state (i.e., state of solution) and a phase-separated state, as described by Miyata, et al. in "Optical Recording Material Using Polymer Blend" in the Collected Papers for The First High Science-Technology Exhibition, P. 31 (1986) published by Tokyo Agricultural and Technological Univ., 1986. However, it is practically impossible to record sufficient amount of information by using only the polymer blend because the polymer blend itself has a low absorption for a light such as a laser beam. Hence, the polymer blend appears to be hardly put into practical use.